Anthropogenic influences on ecosystem processes in a tropical estuary: feedback connections and environmental management targets - a case study of Dong Ho Estuary, Kien Giang, Vietnam

Abstract

Estuaries play an invaluable role in the transformation and cycling of materials as they move between land and sea; including anthropogenic materials. Increasingly, human activities are delivering material loads that risk overwhelming the normal functions that these ecosystems provide. In this context, the objective of this study is to assess key biogeochemical processes to understand the influence of anthropogenic activities on ecosystem performance in a tropical estuary. The research presented here focuses on a case study area, Dong Ho estuary in Kien Giang province, Vietnam, which exemplifies the anthropogenic impacts and management issues facing most of the Mekong coastline and other similar areas in Vietnam. Located at the south-western edge of the Mekong delta, the Dong Ho estuary is subject to material inputs from both local and more remote sources; making it vulnerable to degradation and functional loss. Further, as a major food producing area of Southern Vietnam, Kien Giang province and its coastal ecosystems, such as Dong Ho, are under growing pressure from the national government to increase food production (typically rice and prawn aquaculture) but, at the same time, maintain high ecological standard of performance. In addition, the Dong Ho area is witnessing significant increases in population and planned tourism development. Against this backdrop the Dong Ho estuary and its management represent an opportunity to understand the interactions and complexities of anthropogenic activities on ecosystem function and sustainability. In this context, the current study sort to identify the main anthropogenic factors and potential pollutant sources to the ecosystem; define the key physic-chemical and biological factors affecting ecosystem function; identify loads of key materials into the estuary that influence carbon and nutrient stocks in the water column and benthic sediments; measure key biogeochemical processes such as primary production, internal benthic nutrient fluxes, denitrification that underpin ecosystem function; assess if the estuary is retaining materials or simply acting as a pipeline to the sea; and, in view of the new understanding of system behaviour, consider the implications of current inputs and ecosystem function for management and future sustainability of the Dong Ho estuary.Results from fieldwork and experimental observations showed that the Dong Ho estuary is a strongly heterotrophic ecosystem relying on allochthonous sources of carbon to meet its internal carbon budget. At the same time, the estuary captures carbon, nitrogen and phosphorous annually, although nitrogen tends to be exported to the adjacent Western Sea during the dry season. Further, water column nutrient stocks reflect a strong input of materials from the surrounding catchments during the wet season and dissolved oxygen levels during some periods become very low in bottom waters, indicating a move toward eutrophication and ecosystem dysfunction.From a management perspective, results from this study highlight the need for strong consideration of current proposals to further alter the hydrodynamics of the estuary through infrastructure development, and the need to develop key wastewater infrastructure and management strategies to mitigate the augmentative impact of urban discharges on the increasing loads from local and remote agriculture and aquaculture activities.